1. Field of the Invention
This invention relates to uninterruptible power supply systems and, in particular, to power supply systems having plural power sources which normally include a primary power source and a reserve power source. It is specifically concerned with a power flow control system to control the flow of power from plural, independent power sources to an output load to be energized.
2. Description of the Prior Art
Modern electronic systems require extremely reliable sources of power. Examples of electronic systems needing reliable power include computers, data processors, process controllers, and communications equipment. In general, the available commercial power supplied by utilities is not sufficiently reliable to meet the power needs of such equipment. Commercial power is sometimes subject to complete outages or, in other words, complete failure of the power signal. More often, due to inadequate capacity and increasing load demands, it is subject to a condition known as "brownout" which normally occurs during peak demand periods and usually is typically represented by a three to eight percent drop in the magnitude of the available voltage. Commercial power is very frequently subject to magnitude and phase instabilities causing irregular voltage waveforms due to transients induced by the action of various customers who subject the system to sudden electrical loads. These various changes in the characteristics of the voltage supplied by commercial sources may have significant adverse effects on the operation of electronic equipment like computers.
To operate properly, a computer requires a precisely regulated, continuous power signal. The fluctuations it can tolerate from a power source are extremely limited. The computer can probably tolerate momentary spikes and dips in the voltage if the duration is only a few milliseconds. It can probably also tolerate a slight brownout for a short period of time not exceeding 100 milliseconds. Should the brownout equal perhaps an eight percent voltage drop and last beyond 100 milliseconds, it is possible that the computer could malfunction or shut down. Should the brownout voltage drop be ten percent or greater for a period of time exceeding 100 milliseconds, there is a very definite possibility that processing errors may occur requiring partial program reruns. In instances of severe brownout where the drop in voltage magnitude may exceed 25 percent, the computer will probably go into total shutdown, and if the power drops abruptly the computer will also terminate operations probably with a high possibility of component damage and of adverse effects on the integrity of the stored data.
It is therefore apparent that for safe, reliable operation a computer needs a source of continuous, regulated power having very stable characteristics. Since the power normally supplied by commercial power companies does not possess the necessary stability for safe operation of computers, it is customary to supply the power to computers with uninterruptible power supplies. Uninterruptible power supplies include plural sources of power which usually operate in conjunction with each other to provide a continuous power output to some load to be energized. The plural sources generally include a commercial AC power source and an auxiliary independent source of power to supplement or substitute for the commercial AC power as required in order to supply the necessary continuous and stable power input to the load to be energized.
One simple method of providing an uninterruptible power supply is to connect a charger and rectifier to the commercial AC power source. The rectified output is connected in parallel with a reserve battery-type power source and both sources are used to drive an inverter circuit from which the power signal to energize the computer is derived. This power supply arrangement requires few parts and is relatively simple in design. However, there is no redundancy to provide power to the load should the inverter fail. It is generally recognized that inverters are fairly reliable but they are generally incapable of handling rapid load change demands which generally result in overcurrent, short circuits, or in-rush current conditions. A rapid load transfer can create signal disturbances which will destroy the inverter.
To avoid these problems, uninterruptible power supplies have been designed where the primary commercial power source and the reserve power source are connected in parallel. Both the primary power source and the reserve power source are continuously operated and both sources contribute to the energization of the load. This is a completely redundant system and should either power source fail the results are not apparent to the load which is continuously energized. Such an uninterruptible power supply system typically uses a ferroresonant transformer with two input primaries which are coupled to energize a single secondary. Through the use of properly designed high reluctance shunts, the two power sources do not transmit power to each other. Both power sources cooperate to share the load's power needs. The disadvantage of this particular arrangement is the expensive transformer design of a ferroresonant transformer having carefully designed high reluctance shunts and symmetrical construction to permit the two power sources to share the load.
An uninterruptible power supply arrangement which permits use of a less expensive power coupling arrangement is commonly known as the "transfer type" of uninterruptible power supply. Generally, a commercial AC line power source and a DC voltage energized inverter power source are connected in parallel to a switching mechanism which alternately couples one or the other of the two power supplies to a load to be energized. This power supply design advantageously eliminates the need for an expensive ferroresonant transformer and provides redundancy to provide a substantially uninterruptible power to the load. However, the need for switching devices reduces the reliability of the circuit. In addition, the auxiliary power source comprising the inverter must be synchronized in frequency with the AC power line signal which requires complicated synchronizing circuitry. The switching action must be sufficiently fast to handle the transition of a load from a failed AC power line to the reserve power source or inverter circuit without inducing damaging transient signals into the circuit. The switching must disconnect the failed power source so it does not become a load for the active power source. Additionally, should the inverter fail, it would only be discovered at the time it is needed most, i.e., at the moment of a power transfer.
It is therefore an object of this invention to improve the dynamic load transfer characteristics of an uninterruptible power supply and eliminate synchronizing requirements between two independent power sources.
It is another object to reduce the size and simplify the control circuitry required for an uninterruptible power supply.
It is another object of the invention to prevent the flow of energy from one input of the uninterruptible power supply to another input of the uninterruptible power supply without the necessity of complex switching arrangements.
It is yet another object of this invention to minimize signal disturbances transmitted to the load during the transfer from one power source to another.